1. Field of the Invention
This invention relates generally to RF plasma deposition systems, and more specifically, to features for providing improved Plasma Enhanced Chemical Vapor Deposition (PECVD) systems for depositing conductive films in RF plasma reactors.
2. Description of the Prior Art
In the past, RF plasma reactors have been used extensively during various processing steps in the fabrication of semiconductor devices, such as photo resist removal, etching of silicon compounds, and more recently, for the deposition and growth of conductive and dielectric films. Plasma technology (PECVD) offers the advantages of being clean, uniform, easily regulated, and well-adapted for automation. In particular, large amounts of research have been directed to developing production quality RF plasma reactors for deposition of conductive films such as doped polysilicon, and conductive and expitaxial films.
Originally, RF plasma reactors for use in the deposition of films during semiconductor device fabrication, called "glow-discharge reactors", were comprised of an evacuated quartz reaction chamber, the inside of which was a radiantly-heated semiconductor substrate holder, and a source of RF power through a two-turn coil surrounding the reactor immediately above the substrate holder. The reactant gases, the elements of which determine what type of film will be deposited, were usually mixed prior to being introduced into the bottom of the chamber.
The deposition procedure consisted of placing the workpieces on or in the holder, evacuating the reaction chamber, and initiating the plasma field (a partially ionized gas induced by a strong electric field, and comprised of neutral species, ions, and electrons) above the substrate by introducing the reactant gas into the RF field in the reactant chamber. In this manner, the reactant gas is ionized, or compounds can be formed by introducing subsequent reactants, for depositing the desired ions, compounds, or neutral molecules on the exposed surface of the wafer. The thickness of the film is controlled by varying, independently of one another, the temperature, pressure, concentration of reactants, and strength of the RF field.
A major problem with the original RF plasma reactors was the very limited number of workpieces that could be processed at one time. Eventually, the capacity of RF plasma reactors equalled or exceeded that of morre conventional thermal deposition systems. The inside of the reactor tubes consisted of a plurality of conductive plates, electrically isolated from one another by quartz (or similar non-conductive materials) spacers. RF power was applied to alternate conductors to produce a plasma field in the space between adjacent conductors. On the side of each conductor were pockets in which semiconductor wafers were placed. In some larger systems, in excess of 90 wafers could be processed in a single reactor tube. An exemplary system is described in U.S. Pat. No. 4,223,048, issued Sept. 16, 1980 to George M. Engle, Jr., a co-applicant for this invention.
The larger, production sized RF Plasma Reactors operated on the same principle as the earlier type PECVD systems. However, it was often impossible to run the reactors for more than a very short period of time, during which only a small deposition could be produced on the semiconductor wafers. This problem became especially prevalent when RF plasma reactors were used in PECVD of conductive films, and resulted largely from the thermal deposition of the conductive material on the spacer means. If the RF reactors were run for relatively longer periods of time, the deposited film would accumulate on the insulative spacers between adjacent conductive plates. As a result, and especially when depositing conductive films, adjacent conductive plates would be shorted together by the accumulated conductive film on the spacers. This would cause the plasma field to break down and the deposition process to stop. Even where a single deposition run could be completed without failure, system dismantling for cleaning raised costs and limited throughput.
The problem of curtailed run times and shorting together of the conductive plates prevented the most advanced RF plasma reactors from being used in efficient, production rate PECVD systems.
There existed a need to provide a means for isolating and preventing the shorting together of adjacent conductive plates in RF plasma reactors, so that the plasma enhanced chemical vapor deposition (PECVD) of conductive or other films onto semiconductor wafers can be done at a production rate in production lot sizes, and so that a multiplicity of runs could be effected without the necessity for dismantling and cleaning the system.